1. Field of Invention
The present invention relates to glass powders in the Bi2O3—ZnO—B2O3 system. Such glasses have low melting points and provide good flow characteristics with low or tunable crystallization tendencies.
2. Description of Related Art
Microelectromechanical systems (“MEMS”) devices are microscale machines that perform work or measurements such as an accelerometer, rate sensor, actuator, pressure sensor and the like. Signal lines electrically connect the MEMS device to a microprocessor and/or to other circuitry. MEMS devices are plagued by the possibility that moisture, dirty air, dust and other foreign matter may enter the mechanism and cause premature failure or otherwise impede the operation of the MEMS device.
Sealing glass compositions used in MEMS device fabrication are typically applied using screen printing techniques, in which the sealing glass composition is deposited in the form of a paste that contains a particulate glass frit material (including crystalline additives for expansion modification), a thixotropic binder, and a solvent for the binder. The proportions of glass fits, additives, binder and solvent are adjusted to allow screen printing of a controlled volume of the paste on a designated bonding surface of one of the wafers, typically on the cap wafer. After drying, binder burn out (BBO) and pre-glazing, which removes all of the organic components from the glass frit bonding paste, the cap and device silicon wafers are aligned and then mated so that the glass frit particles contact complimentary bonding surfaces. The wafers are then incrementally heated to remelt, flow and impart wetting of the wafer surfaces by the glass frit so that upon cooling, the glass frit material re-solidifies to form a substantially homogeneous glass bond line between the wafers.
In MEMS bonding low firing temperatures are required to protect the properties of mechanical devices fabricated on MEMS wafers. In many of these applications, leaded glasses have been used as sealing glasses where very low firing temperatures are desired (less than 500° C.). However, environmental concerns typically rule out leaded glasses. Conventional lead-free glass powders do not flow sufficiently at temperatures less than 500° C. Phosphate and vanadate glasses in some situations have softening temperatures suitable for flow in this temperature range. However, such glasses are either not resistant to water attack (phosphate glasses often are water soluble) or crystallize too much before fusing and flow of glass powders. In the photovoltaic industry, there exists a need to develop glass based durable seals between glass plates to enhance the service life time of the photovoltaic devices that are being encapsulated from moisture attack. Currently the crystalline silicon solar cell is encapsulated with Ethylene Vinyl Acetate (EVA) polymer in between the glass superstrate and backsheet. The state of the art photovoltaic devices are at present encapsulated with organics as edge seals between glass substrates (for rigid cells). The desired lifetime for these cells is 25 to 30 years with the power output not to decrease below 70% of its initial value at the end of 30 years in the use environment. Often encapsulation with organic seals will not be impervious to moisture for this long period. Therefore more durable low-temperature glass based hermetic sealing technologies have to be developed to realize this desired lifetime with some certainty. Low sealing temperature is required to avoid unduly heating the solar cells being encapsulated. A similar need exists for low temperature glass based sealing technologies for sealing Organic LED devices. Similarly in the building industries there exists a need to replace organic based seals in windows with glass based durable seals to provide superior vacuum insulated glass windows.
In the photovoltaic industry the sealing glass compositions can be applied by a number of techniques such as screen printing, extrusion of pastes onto the glass substrates, ink jet printing (for thin layers), pad printing techniques, and tape casting method. The sealing glass can be either preglazed before the sealing step or can be directly sealed between glass plates in one step. The firing method can be either in conventional furnaces as well as by selective heating methods such as laser sealing, IR or visible light lamp sealing, induction sealing as well as microwave sealing.
Similar methods of paste applications and firing methods can be used in hermetic sealing of windows in the construction industry.
Accordingly, improvements in the art of low melting, high flow glasses, are needed.